Feature Story

NOR Flash Standard Meets Instant-On Expectations

TORONTO – Jedec's new xSPI standard for non-volatile memory (NVM) devices is aimed meeting the demands of instant-on applications while also maintaining performance standards for NOR flash sitting outside of the SoC.

Adesto Technologies, known for its small, ultra-low power NVM products, is claiming to be first out the gate, having been working on products and collaborating on the standard for the past three years. In a telephone interview with EE Times, Adesto Chief Techology Officer Gideon Intrater said its eXecute-in-Place (XiP) EcoXiP product family takes advantage of the new Jedec standards to give customers such as system developers and controller designers assured compatibility with controllers and peripheral devices that should accelerate adoption.

Developed by a task force comprised of representatives from most NOR flash device manufacturers and several PC and microcontroller companies, the xSPI standard establishes mechanical, electrical and transactional guidelines for developing high-throughput octal devices. Although Jedec has been a defining organization for NAND flash, said Intrater, until recently NOR flash has been somewhat of a "wild west” in terms of standards, which has led to divergent products for enabling communications between host controllers and memories and confusion for controller designers.

NOR flash is "old in the tooth,” he said, but it's made progress in terms of process technology and the amount of bandwidth that you go can get out of devices has improved dramatically in the last decade. Recently emerged applications such as the Internet of Things (IoT) and automotive infotainment systems have created a great deal of demand for embedded devices that need more program memory than what can be implemented economically on-chip using embedded flash or SRAM. Intrater said Adesto's EcoXiP eliminates the need for on-chip embedded flash and the need for on-board flash to store firmware. It doubles processor performance, lowers system power consumption and reduces system costs compared to quad SPI devices.

Intrater said IoT and embedded devices such as wearables, medical monitors, POS controllers and other connected embedded systems now must support higher levels of functionality, new wireless protocol stacks and advanced software. They must be designed to handle more intelligent local data processing, so they need more program-memory than what can be implemented economically on-chip using embedded flash or SRAM, and less than what is offered by the smallest DRAM devices.

Adesto's EcoXiP achieves low latency and high throughput using a high-speed interface and a highly accurate pre-fetching scheme. The company noted that EcoXiP numbers assume 16-byte Instruction Cache line-size and an average of 3.84 line fetches per IC miss.

"A lot of companies are moving away from having a huge amount of NOR flash inside their SoCs,” Intrater said. "They're building SoCs where the flash is sitting outside. This allows them to build devices with more aggressive process technologies.” For this approach to make sense, the external interface needs to provide a level of performance that's not slower than on-chip.

The xSPI electrical interface can deliver up to 400MBytes per second raw data throughput. Adesto's EcoXiP claims to deliver 2.4 times the CPU performance compared to existing quad devices and established new standards for an octal interface; eight parallel data (IO) lines are used in xSPI to further increase system performance by transferring more parallel bits in each clock cycle.

Intrater said XiP shows a lot of promise for mid-sized IoT systems and makes it possible to have more program memory than what is available on-chip, flexibility in size of the program memory without the need to re-spin the SoC, and the ability to turn off the power to most or all of the SoC when not needed.

Adesto is sampling a 32Mb device now, with a family of densities planned. The immediate market is for applications that require "instant on” capability, such as rear-view cameras and GPS in advanced driver-assistance systems (ADAS), as well as IoT devices that wake up to send data and shut down again. "There's instant on for security and the instant on for convenience,” said Intrater. "We don't like waiting as consumers.”

Jim Handy, principal analyst with Objective Analysis, said many IoT applications need a richer complement of memory than most MCUs offer. "NOR flash makers can do well by supplying chips to satisfy this need. EcoXIP appears to be a good solution to the problem,” Handy said.

Adesto said its xSPI-enabled systems will offer designers power savings and performance gains at lower cost than conventional on-chip memory SoCs.

NOR flash is going down a path that was paved by EEPROM, Handy said. "You used to have serial and parallel EEPROM and then it boiled down to just being the serial because the serial took fewer bonding pads, and we see the same kind of thing going on with NOR flash,” he said.

Handy said serial channels have seen a great deal of research over the last 20 years. "They've become extraordinarily fast,” he said.

Adesto is likely first to market with a product based on the Jedec standard, Handy said, because it was already well ahead of the game before collaborating on the specs. However, he said, the Jedec standard doesn't always guarantee success. "There are Jedec standard devices that do very well business-wise, but there are also Jedec standard devices that do poorly,” he said. "There are non-standard devices that do very well and there are standard devices that do poorly.”

The new xSPI standard is helpful in that in open up opportunities for other manufacturers to produce something that is pin compatible. "Companies that don't want to get themselves stuck in a sole-source situation will be to get multiple vendors simply because it's a Jedec standard,” he said. "That's where I think this thing is headed.”

NOR flash serves a role as code storage for applications, which makes it ideal for devices such as NEST thermostat, rather than NAND, because they can be designed for a lot lower cost. "IoT is made to order for NOR flash,” Handy said. Cypress likes to point out that automotive makers like NOR flash because it makes the dashboard to start up as soon as get keys in the car. "We've had 70 to 80 years of dashboards waking up and being able to function immediately,” Handy said.

Handy said one of the difficult decisions facing designers is whether they can put all code they need into the internal NOR of an MCU, or whether to put it in an external part, which costs more. "There are micro-controllers that have large NOR flash inside but they're extremely expensive,” he said. Products like those from Adesto make it easier for designers to put big programs into their systems because the cost penalties aren't as high. "It works to Adesto's advantage that people are focused on putting a lot of code into small controller-based applications,” Handy said.

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